1. Field of the Invention
An aspect of the present invention relates to at least one of an electrochromic display device and a method for manufacturing an electrochromic display device.
2. Description of the Related Art
In recent years, a need for an electronic paper as an electronic medium that replaces a paper is growing and its development has been conducted actively. For means for realizing such a display system, development of a self-emitting display technique such as a liquid crystal display or an organic EL display is proceeding and a part thereof is being produced. Meanwhile, a reflection-type display technique with low electric power consumption and a less visual fatigue is promising as a next-generation electronic paper display technique.
For a reflection-type display technique, for example, a reflection-type liquid crystal display technique using a cholesteric liquid crystal has been devised. However, such a display technique utilizes a selective reflection or involves a large number of substrates, so as to be poor in reflectivity, contrast, color saturation, and a color reproduction range, and provide a visibility that is far inferior to that of a “paper”.
Furthermore, an electrochromic display technique composed of an organic electrochromic material provided with both a high color reproducibility and a display memory property is attracting much attention in a reflection-type display technique. A phenomenon in which an electric voltage is applied to cause an oxidation reduction reaction reversibly and change a color reversibly is referred to as “electrochromism”. An electrochromic display device is a display device utilizing color appearance/color disappearance (which will be referred to as “coloring/discoloring” below) of an electrochromic compound that causes such a phenomenon of electrochromism. Such an electrochromic display device is a reflection-type display device, has a display memory property, and is able to be driven at a low voltage, and hence, study and development, such as material development to device designing, have widely been conducted as a potent candidate for an electronic display technique for an electronic paper application.
An electrochromic display device is able to develop various colors depending on a structure of an electrochromic compound, and hence, is expected as a multicolor display device. For multicolor display techniques, Japanese Patent Application Publication No. 2009-163005 and Japanese Patent Application Publication No. 2010-033016 disclose configurations in which a plurality of layers of display electrodes and an electrochromic coloring layer are laminated on one display substrate.
Japanese Patent Application Publication No. 2009-163005 discloses that an intermediate display electrode provided between a display electrode and an opposed electrode is formed by an indium tin oxide (which will be described as ITO below) fine particle. For an electrically conductive film formed by an ITO fine particle, it is known that a volume resistivity thereof is 2-5 digits higher as compared with that of an ITO electrically conductive film formed by a vacuum film formation such as a sputtering method. A display electrode layer with a low electrical conductivity is a cause that produces an in-plane non-uniformity in a display image as a surface area of a display electrode is increased. Furthermore, as a film thickness of an ITO fine particle display electrode layer is increased to improve an electrical conductivity, a cause is provided that causes degradation of a reflectivity and degradation of a display image quality.
Furthermore, Japanese Patent Application Publication No. 2010-033016 discloses a display electrode and insulating layer that have permeability into an electrolyte, and further discloses that an ITO film formed by a sputtering method is used for the display electrode. However, an ITO film formed by a sputtering method is dense, has a poor ion permeability, and has a disadvantage in such a manner that a large voltage is needed for a coloring and discoloring reaction of an electrochromic layer provided adjacently to a display electrode that is not directly opposed to an opposed electrode via an electrolyte. That is, there is a problem in such a manner that a coloring and discoloring voltage for an electrochromic layer provided separately from an opposed substrate is increased even if a plurality of pairs of display electrodes and an electrochromic layer are laminated.
Furthermore, Japanese Patent Application Publication No. 2010-282128 discloses a method for holding an electrochromic layer on a porous sheet body having an electrical conductivity and superposing a plurality of porous sheets thereon. However, there is a problem of a display image blur that originates from an increase in a distance from an opposed electrode to each display electrode due to a thickness of a porous sheet per se of an increase of a driving voltage.
Meanwhile, a transparent electrically conductive film formed by a long-fiber-shaped electrically conductive particle has characteristics of a high transmittance of visible light, a low surface resistance, a high distortion resistance or flexion resistance, a capability of being formed by each kind of printing method without needing a vacuum film formation, etc., and study and development have been conducted as an alternative to a transparent electrically conductive film such as ITO. Furthermore, applications to a touch panel, a liquid crystal display device, a solar cell, an organic electroluminescence display device, etc., have also been investigated (for example, Japanese Patent No. 3560333, Japanese Patent Application Publication No. 2009-252014, Japanese Patent Application Publication No. 2011-082092, Japanese Patent Application Publication No. 2009-253016, and Japanese Patent Application Publication No. 2010-118165).
For example, Japanese Patent Application Publication No. 2009-253016 discloses a solar cell using a metal nanowire as an electrode on a substrate but this relates to a dye-sensitized solar cell and is a metal nanowire as an electrically conductive layer provided on a substrate.
Furthermore, there are problems in such a manner that a silver nanowire that is a long-fiber-shaped electrically conductive particle is generally readily degraded by an oxidation reduction reaction and migration is readily caused due to presence of an ion. As a measure for solving such a matter, Japanese Patent Application Publication No. 2009-127092 discloses a method for preventing migration by applying a plating treatment with another metal atom to a surface of a silver nanowire.
Moreover, a transparent electrically conductive film formed by a long-fiber-shaped electrically conductive particle generally has a weak film strength, and hence, a part of an electrically conductive particle is generally fixed by an overcoat such as a polymer. Hence, an overcoat with no ion permeability is generally used in order to prevent silver migration.